Tree Rings Crack’d?

One day walking from my home to work and crossing a footbridge over the River Yare into the inner sanctum of UEA, I was appalled to see a beautiful line of crack willows, the haunt of many a kingfisher, was being felled. These trees had graced the river bank for around a century (estate records exist of their planting). Apparently increased river flows were undermining the tree roots and the trees were now considered a danger to canoeists, fishermen and passing pedestrians alike. I spoke to the chief fellah (feller?) and asked if he could keep for me a slice through a tree trunk that showed the least amount of damage and was as close to the ground as possible. Crack willows are well named sine they develop deep fissures, and I wanted the least damaged specimen. I planned to get one side of the slice polished and its tree rings counted and annotated. I thought it would fit well at the entrance foyer of the School of Environmental Sciences, especially if we could identify and highlight the years when UEA was founded and ENV moved into its current building, as well as dates of lesser importance like the beginning and end of WW2. I already had a photograph of the tree-lined river bank to accompany the tree slice that would inform visitors of the former beauty of the river banks.

A few days later, whilst crossing the footbridge again, I was presented with a tree slice around 10cm thick and 60 cm across, within which the tree rings were prominent and rather thicker than I had expected. I had it polished and set about counting down the tree rings to discover how many there were and the location of particular years.  

While I was tree-ring counting, Keith Briffa of CRU (= Professor Tree Ring) came into my office. His comments, as far as I remember them were:

1) Pity the tree hadn’t been an oak, because that would have been ‘much more useful’ and,

2) (Looking at my pencil marks where I had dated individual tree rings), ‘You appear to have missed some’ (I hadn’t).

He showed little interest in what the tree slice was intended for (perhaps if it had been from an oak?)

Tree rings as temperature proxies?

Part of the reason for my interest in tree rings was that I had always been sceptical that the width or density of tree rings could record varying temperatures and particularly reflect some average global temperature. I therefore questioned if they could act as reliable temperature proxies for past times (a view I still hold). I did learn over the years that it wasn’t as simple as I had originally thought, that the data came from locations that were marginal for tree growth and where the variable most likely to be controlling growth was temperature and not water supply or sunshine amounts. But still a question remained for me: could those special locations be representative for global climate fluctuations? Also, surely the global variations would affect those special locations (otherwise why bother?), potentially making trees growing there more or less sensitive over time to climate variables other than temperature. The more I examined the arcane practices of dendroclimatology, the more it seemed to be voodoo science. But I never could express this to its practitioners in CRU, because to do so would strain relationships, perhaps beyond repair (and I still had to work with them).  So I kept my own counsel.

Climategate changed things. Strain increased between CRU and myself, but with discussions raging everywhere in and outside the university about ‘hiding the decline’ I felt less constrained to express my doubts and thoughts about tree rings (and other proxies). I also began to think outside the box about how to employ tree ring variability to unravel the various and variable controls upon the thickness of those tree rings. I came up with two strategies that might show promise.  

The first strategy was to pick upon a single widely grown tree species (probably a conifer) and compare its reaction (by tree rings) to varying climatic variables across the U.K. from high altitudes to low, from the wetter west to the drier east, and also taking into consideration changing levels of light intensity sunshine and or cloudiness. I was reasonably certain that this would already have been done, but was assured that it hadn’t been, or not with the rigour that I was proposing.

For my second strategy, I recalled Keith’s reaction to my crack willow slice. Why would he prefer an oak? Now, I think it was because oak tree rings are very narrow and, for a 60cm wide slice, there would be a longer history to unravel. But when I initially was thinking about it (probably wrongly) I wondered if it might be due to the obvious fact that living alongside a permanently flowing river, the willows were never afflicted by a shortage of water, thus this variable could be excluded as influencing annual tree ring width. Thinking more widely, I thought that different types of tree might react to different climatic variables to different extents. Consequently, sampling many different tree species from a single locality might reveal these different influences (not just temperature) and thus show their variability over time with, say dry years, having a greater effect in some species than in others, whereas a different suite of species might be more affected by late frosts, and so on. I proposed that this sort of study might be suitable for an undergraduate thesis, sampling from different trees within the UEA campus. However I could never conjure up enough interest within CRU to supervise such a  project, nor with any undergraduate. So it remains a future project for someone. I was surprised at the apathy from CRU, because with a relatively small investment of their time they might have gained better understanding of what controlled a tree’s response to different climatic variables and so potentially improved their proxy records. But hey-ho.

The decline controversy

After Climategate 2, I became embroiled in a blog war over whether or not I should heap further calumny upon my former colleagues within CRU. One of the main bones of contention concerned Phil Jones’ email where he wrote:

“I’ve just completed Mike’s Nature trick of adding in the real temperatures to each series for the last 20 years (i.e. from 1981 onwards) and from 1961 for Keith’s to hide the decline.”

The words ‘trick’, ‘hide’ and ‘decline’ caused sceptics worldwide to go apesh!t. Gallons of printer’s ink were spilled trying to manufacture nefarious explanations for these words, especially concluding that ‘the Team’ were trying to disguise the fact that global temperatures had fallen. Nothing could be further from reality. Phil Jones, Keith Briffa, Tim Osbourne and others complained bitterly that the Climategate emails were being interpreted out-of-context and that Phil Jones’ ‘hide the decline’ email was a perfect case in point. I agreed with them on this one point.

In the email the word  ‘series’ referred to three different attempts to reconstruct average temperatures of the past using proxies, mostly from tree rings.

‘Mike’s Nature trick’ was to add the 19th and 20th century instrumental temperature record to the end of Mike’s proxy record, so emphasising (exaggerating?) the ‘unprecedented’ global warming. Jones was employing the same device (or ‘trick’) on a diagram he was constructing that used the three different proxy records (or series), including one from Briffa that employed a new technique using summer wood density (rather than tree ring widths) as the proxy. When applied to past centuries this new technique gave results that tracked those using tree ring thicknesses and so was accepted as a valid indicator of past temperatures. That is up to around 1960 when the Briffa technique goes amok. Temperatures determined by thermometers (and some averaging chicanery) showed a steep rise post 1960, but the Briffa proxy temperature curve displayed an equally steep decline. So the ‘decline’ in Phil’s infamous email did not refer to instrumentally determined temperatures, but to temperatures determined from summer wood densities which were considered spurious. Jones resolved his problem of constructing a diagram showing what he wanted to show by omitting the Briffa curve after 1940, so ‘hiding the decline’.

After the Climategate email release the CRU part of ‘the Team’ were up in arms, loudly protesting their innocence. They argued, with some justification, that they were only trying to tidy up an already overly complicated diagram by removing part of a proxy curve that was obviously wrong. I understood that Keith Briffa strenuously objected when it was suggested that his entire proxy curve be removed from the diagram and still was unhappy when the last part of his curve was obliterated. He wanted his work acknowledged and used.

At the time I totally agreed with CRU that criticisms that they were hiding a fall in average global temperatures were incorrect. They were instead hiding what they considered spurious data where tree-ring derived temperatures did not replicate their interpreted unprecedented steep rise in thermometer-determined temperatures. This was bad enough because it also hid the possibility that the tree-ring proxy method could have ‘gone rogue’ at other times and so the various reconstructed temperature curves may have omitted previous warm periods (like the Medieval Warm Period).  

In the intervening years my judgement upon this matter has changed. Removal of the most recent parts of the proxy curves were indeed hiding an apparent decline in temperatures because it was a reconstructed temperature curve that was being hidden, not a curve displaying tree-ring widths or densities. The fact that they could argue that only spurious data was being removed was not pertinent. They could not demonstrate that earlier parts of their proxy-determined temperature plots were not free of the same ‘errors’.  

I also found Keith Briffa’s reluctance to use the crack willow and give support for a proposal to examine different tree species on the UEA campus difficult to defend. The reason why tree-ring wood densities are seemingly controlled by temperature at some times but not at others is still unknown. In keeping with the mantra that recent high temperatures have been caused by human release of carbon dioxide ‘pollution’ the recent deviant behaviour of tree-ring densities has also been ascribed to other human changes – global dimming, pollution either acting directly or indirectly as when the protective cover of stratospheric ozone is destroyed. But, as far as I know, this is all conjecture with no proof. What cannot be admitted is the possibility that whatever the cause of the anomalous tree-ring density and temperature relationship, it could have acted before the period when past temperatures were available from thermometers. That would open up an entire can of worms that dendroclimatologists seemingly could not face.

The fate of the tree slice

I suppose many of you are eager to learn what happened to the tree slice. I am sorry but I can’t tell you, not because I’m sworn to secrecy but because I just don’t know. I left UEA without it being installed, and last I saw, it was in the office of ENV’s Head of School, gathering dust. I am informed that it is not on display anywhere. Dendroclimatology was a genuine attempt to extend our knowledge of past climates but attracted ‘bad vibes’ for the School that I doubt have entirely gone away. Perhaps a display of tree rings would have stimulated too many bad memories. Most likely the tree slice, still with my pencil marks upon it, resides in a storeroom somewhere, unloved, somewhat like the fate of the biblical artifact in Raiders of the Lost Ark.

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May 3, 2021 at 02:59AM

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